The ever-increasing performance capabilities and requirements of integrated circuit chips requires an increasing number of interconnections between chips at the substrate level. To fulfill this need, multilayer ceramic module technology was developed. The substrates for these modules had many (up to 23) layers, and each layer began as a part of a continuous cast sheet of ceramic material, which was cut into pieces 175 mm square, then punched with holes at high speed so that electrical connections could later be made between layers. Conductive paste was then extruded onto the green sheets through metal masks, forming a wiring pattern unique to a given layer. Stacks of these sheets, with the required configurations of conducting line and insulating layers, layer-to-layer connections, and reference and power planes, were laminated together and trimmed to form individual modules, which were then fired in a furnace to harden the ceramic. Finally, the upper surface of the module was plated with the chip sites. After electrical testing of the ceramic modules and the attachment of the input/output pins, a number of chips, at most nine but more typically six, were joined to their plated sites using Controlled Collapse Chip Connection (C4) joints. Up to 10 m of wiring could be contained within these 4 mm thick multilayer modules.
A more recent alternative to the multilayer ceramic module is the multilayer printed circuit board (PCB) that is fabricated by interleaving the cores with additional sheets of prepreg and copper foil. Lamination, hole drilling, photolithography, and plating processes are repeated to construct a multilayer composite. Although the process flow for the fabrication of a multilayer board is fairly simple, even multilayer circuit boards with just a few layers may require a hundred process steps. These are necessary to ensure interfacial cleanliness, to develop adequate adhesion of copper circuitry to the base resin, to achieve resist adhesion and proper resist performance, and to maintain the integrity of the inter-plane joint in the plated through-holes. As a consequence of the complex sequence of highly diverse processing steps, the circuit board materials are exposed to a number of very aggressive environments. These include aqueous etching/plating solutions, aqueous solutions or organic solvents for developing and stripping photoresists, various cleaning and degreasing agents, and high-temperature soldering operations. Each process has a high degree of complexity with the possibility of many types of interactions.
Clearly, one can see that it is difficult to control the quality and manufacturing process in both the ceramic module and the multilayer PCB, and that the inherent complexity of the manufacturing process mitigates against low cost and high reliability. A better method of making a multilayer high density interconnect substrate would be a significant contribution to the industry.